1. Field of the Invention
This invention relates to apparatus for selectively exposing an area on a printed circuit board, for example, to permit the bonding of wire or the like to said exposed area.
2. Prior Art
With the advent of high density wiring on printed circuit boards, for reasons of economy and reliability, it is desirable to utilize automatic wiring apparatus to route wires from a feeding unit for bonding between selected points on the printed circuit board surface. The apparatus may bond either single or multiple stands of wire, and in the above-referenced, co-pending application, for example, there is particular emphasis placed on automatic routing and bonding of pairs of twisted wire.
One of the difficulties encountered in using such automatic wiring apparatus is that as the number of wires connected on a side of a printed circuit board increases (increased wiring density) the bonding sites for the wires become covered by previously routed wires. This is particularly significant where the wires are constrained to flow in rows and columns with the result that there may be a very large number of wires covering particular bonding sites.
Typical of the prior art solutions to the problem of clearing the area to which a wire is to be bonded is shown in the article entitled "High Density Bonding Tweezers" by J. J. Steranko found in the IBM Technical Disclosure Bulletin, Vol. 10, No. 5, October 1967 on pages 630 and 631. According to this, a sheath in the form of a pair of tweezers is used to probe toward the selected contact area. As the bonding site was contacted, the tweezer pairs are moved apart to spread any covering wires and to permit a bonding head to move into contact with the wire to make an ultrasonic bond. The difficulty with this wire maze penetrating device was that it is not readily susceptible to automation, but rather requires manual activation to assure that all of the wires are cleared from the bonding site area in a manner which will not cause damage to the board.
In providing automatic maze penetrating apparatus, it is necessary that enough pressure be provided to accomplish penetration of the wires on the board and spreading thereof, while at the same time preventing any damage to the board surface. If, for example, an automated wire routing mechanism included a maze penetrating apparatus which always descended a prescribed distance from a fixed reference, because of the warpage that exists in a board and the variations in a boards thickness, it is possible that either the maze penetrating tip may pierce the board surface and rupture it as it spreads to clear the defined area or in the alternative, the maze penetrating apparatus may not penetrate all the wires on the surface of the board, so that as the spreading of the tips occurs, some wires remain over the bonding area.